Just found another potential reason of the "oil/dust" problems. The coating on the lower part of the shutter courtain gets scratched off as it moves (maybe sweeped is a better word).. I first noticed it in my friend's D7000 with a higher shutter count - it's pretty evident. In my D600 it's only noticable if I look at a right angle. There are shiny(-er), curved stripes on the matte coating.

D7000 shutter scratch

D600 shutter scratch

Anyone else have it? I wonder what Nikon would say - "normal wear"? "Sand got in"? Sent them those pictures today.D700 shutter is made of different material - no problems there, D300 and D90 shutters look similar to D600, but I haven't found any scratches either.